U.S. Pat. No. 5,225,713 describes an apparatus which permits the start of a vehicle to be prevented when its use is not permitted. This apparatus is controlled by the DTMF frequencies generated by a cellular telephone when the user dials a number. The apparatus compares the frequencies generated with a sequence of pre-recorded frequencies and decides in response to this comparison to block or unblock the starter means of the vehicle.
The user is thus obliged to dial his code on his cellular telephone each time the vehicle is started, which is annoying.